More and more electronic applications are requiring distributed power architectures where the current requirements of the electrical loads are requiring the power supplies to be moved as close to the load as practicable. Instead of the single power supply which would accept ac line voltage and produce a dc or ac output voltage for use by an entire electrical system, today's ultra-fast electronics and electrical components often require their own power supply to accommodate the high transients in their load currents. This new concept in power systems is often referred to as a "distributed power architecture." This type of power architecture can be implemented by means of a system rectifier which converts the ac line current into an unregulated or slightly regulated dc voltage, and numerous "point-of-load" power supplies. The point-of-load power supplies accept the dc voltage from the rectifier and produce a highly regulated dc voltage which is able to accommodate very large current transients (large di/dt).
The point-of-load power supplies need to be small, have a high power density, and be mountable on the circuit boards near the load. In addition, the point-of-load power supplies should be modular to allow two or more to be connected in parallel to supply power to high current loads. This modularity allows a single design to be adapted for loads with varying current requirements. The problem with placing power supply modules in parallel is getting the modules to share current effectively.
Small variances in component values or reference levels will cause one or two paralleled power supplies to supply the majority of load current while some of the remaining modules supply relatively little, or no, current. This disparity in load currents causes the modules supplying the majority of the current to wear faster due to the increased thermal stresses, leading to premature failures in the field. Accordingly, several methods have been tried to force parallel power supply modules to share load current evenly.
A method of current sharing involves master/slave schemes where the "master" module provides the control information used by each of the "slave" modules. One such master/slave scheme is described in U.S. patent application Ser. No. 09/350,840 by Brkovic where each of the modules is capable of acting as either a master or a slave, and upon a failure of the master module, one of the slave modules becomes the master. This system prevents the single point of failure problem associated with other master/slave schemes and prevents the need for two types of modules. In the instance where a master module fails, however, there could be a problem if the new master module's control signal does not closely approximate the previous master's control signal because it had been bypassed as a slave module. This could cause the output voltage of the power supply to move outside the acceptable range potentially causing the power supply to shut down.
The same problem exists for current sharing schemes which place all of the parallel modules control signals on the current share bus and then "or"/"and" the signals together to obtain the "master" control signal used by the parallel modules. In this scheme there may not be a specific module designated as the "master" but the module with the largest or smallest duty cycle is respectively, effectively the master since the master control signal obtained after the "or"/"and" function will track the longest/shortest duty cycle. The modules with the different duty cycles need to have their local control signals regulated to ensure that they are close to the master control signal.
Accordingly, what is needed is a circuit and a method for approximating the output voltage feedback in the controllers of power modules whose local control signal is not being used so that their local control signal approximates the "master" control signal which is either generated by the master module or derived by a logic function.